Cellulose pulp is normally manufactured from lignocellulosic starting materials, such as different types of wood for example.
Wood contains resin in proportions which vary with the type of wood concerned. The presence of mentioned resin has created problems for the pulp manufacturer and/or pulp consumer, irrespective of which pulp manufacturing method is used and irrespective of the type of wood from which the pulp is manufactured. For instance, if paper pulp is produced and the pulp in its final state contains excessive resin, problems will immediately occur in the paper manufacturing process, i.e. in the paper mill. Consequently, there is a general endeavour on the part of pulp manufacturers to remove as much resin as possible from the pulp. In practice, these manufacturers apply with respect to the resin content of different pulps certain guidelines which must not be overstepped. In the majority of cases there is a desire to reduce the pulp resin content to as near to zero as possible. The viscose pulp sector, however, is one exception from the view that resin in pulp will always have a detrimental effect, since it has been found that when using such pulps in the manufacture of viscose, a given low resin content (0.10-0.20% DKM) is beneficial. However, it is necessary to remove the major part of the resin contained in pulp, even when manufacturing viscose pulp, and furthermore the removal of resin in this case is controlled so that said contents will constantly lie within the aforesaid range.
The manner in which the resin leaves the wood and later the cellulose pulp will vary with the method of pulp manufacture required.
In the manufacture of chemical pulp, for instance, part of the resin is released during the actual digestion process and is removed from the pulp when the pulp is washed and screened. The resin content of the pulp is finally adjusted in the bleacing department. It is primarily in the alkali stage (E) of the bleaching process that resin is removed. (ormally, oxygen bleaching (O) will also result in a reduction in the amount of resin contained by the pulp. The final adjustment of the resin content is often effected with the aid of a bleaching agent, for instance chlorine dioxide (D).
When manufacturing high yield pulps, such as for instance groundwood pulps, refiner pulps (thermomechanical pulps) and chemithermomechanical pulps, where the wood is not digested, it is to rely primarily on the bleaching chemicals, e.g. peroxides (P), for the removal of resin from pulp.
The resin problem still remains unsolved even when the major part of the resin present in the pulp is removed therefrom, since the resin is only shifted from the pulp to one or more of the waste liquors obtained when washing the pulp subsequent, e.g., to undergoing one of the aforesaid treatment stages. Waste liquors which contain high proportions of resin are always difficult to handle, due to the fact that the inherent properties of the resin present, inter alia its stickiness, which often leads to blockaging problems of one kind or another. Waste liquor which is rich in resin and is simply discharged to sewage, i.e. released to the recipient, constitutes a load on the environment. Furthermore, the resin thus released has an intrinsic value and can possibly be put to a useful purpose.
In other words, there has long been found a need for removing resin from different waste liquors if possible, and then preferably in the highest possible concentrated form.
Several solutions to this problem have been proposed in the literature, and some of these proposals are applied in practice. One commercially available method requires the waste liquor to be subjected to an ultra-filtration process and the resin concentrate to be subsequently treated in a manner to increase the dry solids content of the concentrate. This method works well in practice. It has been found, however, that a significant membrane area is required in order to achieve the desired resin increase in the concentrate, which involves significant investment costs and also considerable operating costs.
Another method is proposed in the European patent application Ser. No. 0 151 097, according to which a flocculating metal salt (e.g. aluminium sulphate) is added to the resin-containing waste liquor, optionally in combination with an organic polyelectrolyte, and the resultant flocculated product is isolated and thereafter added with a strong inorganic splitting acid to pH 2, or therebeneath, and split at a temperature of at least 70.degree. C., whereafter the resin product split from said product is itself isolated and the resultant split metal sulphate solution is also isolated, this latter solution being recycled in the system if so desired.
Although this method works, it produces, inter alia, a sludge which is difficult to handle, if it cannot be burned separately.
A third method is found described in Japanese patent application No. 53-58002. This method relates to the purification of waste cooking liquor obtained in the manufacture of hardwood sulphite pulp from suspended material. The pH of the waste liquor is adjusted to a value within the range 2.5-5, whereafter a polyacrylamide type chemical is added and a temperature of 30.degree.-70.degree. C. is maintained. The suspended material precipitates out during the treatment process, to form a sediment.
This method is primarily intended for the purification of waste cooking liquors and it is not certain that the method can be applied to other waste liquors.